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First, a little history. 2005 Outback 2.5i AT wagon 135k miles. I have had the intermittent and recurring P0420 (warning: your catalytic converter is dying/dead) for about 30K miles so, in the interim, I may have already done irreparable damage to the cat(s).

Consensus is that cats don't die, they are killed!

When I first went to the dealer with the code, they said "You need a new cat".

Well, yes that may be but without diagnosis of what caused the damage to the cat, I would need yet another new cat in short order.

With the use of a $10 VAGCOM OBDII to usb cable and RomRaider free software, I was able to log real time info on my car's performance.

I am attaching the output zip file in the hopes of getting some help in proceeding with a cure for the underlying cause. If you want to follow this, my posting (username - Juicer) starts at #194 at the above link and in addition, off line, Cardoc's analysis of the data warns:

"Check fuel delivery, pressure and check for vacuum leaks if the fuel pressure is good. Bank 1 is on the end of the fuel system, so if fuel flow is low, it will make bank 1 run lean, which is what is happening. The rear O2 sensors all show a lean mix. Find it quick or you'll be buying a couple CATs due to melting."

Soooo, after 2 years and 30K miles after SOA's "recommendation", monkeying around with spacers, new front O2 sensor, and voodoo, there appears to be a LOGICAL DIAGNOSIS of the underlying cause.

There have been so many posts about the P0420 that I hope to be able to collaborate to make this a sticky for those unfortunates who follow.

In the meantime, in my particular case, I need some guidance on how to "Check fuel delivery, pressure and check for vacuum leaks".

Attached Files

I've found that driving my car too easy produces the code. If I drive a little harder, not much, like keeping it in gear longer in between shifts, downshifting up hills in order to keep the engine speed up, I won't get the code. If the engine is lugging or I'm stuck behind a granny, the code will come up. My car has 312,000 miles on it and the cats aren't melted.

I've found that driving my car too easy produces the code. If I drive a little harder, not much, like keeping it in gear longer in between shifts, downshifting up hills in order to keep the engine speed up, I won't get the code. If the engine is lugging or I'm stuck behind a granny, the code will come up. My car has 312,000 miles on it and the cats aren't melted.

My case is the direct opposite of yours. If my wife drives the car on local streets, code stays off. If I heavy foot it or it's on the hwy for a while the code returns.

My point is that with proper diagnosis of the underlying cause of the code, the mystery of when it occurs is thrown into the light.

Sea Foam or MMO the snot out of the fuel system to clean rails/injectors.

Check fuel pressure.

***135k seems really low for this but injector issues? I've heard of injectors leaking - maybe they can also compromise fuel delivery? Places like http://witchhunter.com/ and http://www.rceng.com/ test, do pre and post flow analysis, and clean and rebuild injectors. I and others I know have used them before.

I also would love to see/learn a proper analysis of this issue and how to log/look at data. I bought that cable but haven't had time to do anything with it besides just plug it in to my computer and attempt to read basic codes.

Oh - and vacuum leaks - spray carb cleaner or starting fluid around the engine bay with particular attention to the intake manifold, vacuum lines and connectors/fittings between them and any vacuum lines associated with the FPR (Fuel Pressure Regulator). If the engine surges as youre spraying then it sucked in that "extra fuel" with the extra air in the vacuum leak....and there's your leak.

Also check Fuel Pressure Regulator - though I've never done that and don't know where it is or how to check one on a Subaru, they don't fail..or should I say get replaced, very often.

I am running this program on WinXP laptop and was stymied for quite some time because I was trying to power off of cigarette ligher converter. RR was unable to make connection to ECU. Use battery or house current... when not driving ;-).

Once you start RR, check to see that the correct port on computer is selected. Should be same as port listed in Computer/control panel/hardware/device manager/port.

Check car defs to see if your car is listed. Mine wasn't but for the sake of logging, I chose a similar car from the same year.

You will also be directed to select paramaters as shown on dashboard page.

The program default is to start/stop logging using the rear defroster switch which prevents you from driving off the road while looking at your screen.

If I exit the program, I seem to have to reboot to reset the port connection.

PS I bought the VagCom cable from FleaBay for $9. Arrived in a week from China.